1. Field of the Invention
The present invention relates to a balanced modulation circuit. More particularly, the invention relates to a balanced modulation circuit which operates on a low supply voltage and which has a wide output dynamic range.
2. Description of the Related Art
The balanced modulation circuit is used extensively in analog communication and as a frequency conversion circuit. FIG. 1 is a circuit diagram of a well-known balanced modulation circuit, shown, for example, in Alan B. Grebene (Exar Integrated System, Inc. Sunnyvale, Calif.), `Analog Integrated Circuit Design`, FIG. 7.3.
In FIG. 1, a first differential amplifier comprises transistors Q21 and Q22, an input resistor R23, and constant-current sources CS21 and CS22, a second differential amplifier comprises transistors Q23 and Q24, and a resistor R25 for output level, and a third differential amplifier comprises transistors Q25 and Q26, and a resistor R26 for output level. An input terminal 21 is connected through a coupling capacitor C21 to the base of the transistor Q21, an input terminal 22 is connected through a coupling capacitor C22 to the base of the transistor Q23, and an output terminal 23 is connected to the collector of the transistor Q26.
The operation of the well-known balanced modulation circuit will be described.
The operation as a frequency conversion circuit is to input a modulating signal expressed by the equation (1) from the input terminal 21, and a carrier wave signal also expressed by the equation (1) from the input terminal 22. EQU Vm=Em COS (.omega.mt) EQU Vc=Ec COS (.omega.ct) (1)
where Em is the amplitude of the modulating signal Vm, Ec is the amplitude of the carrier wave signal Vc, .omega.m is the angular frequency of the modulating signal Vm, .omega.c is the angular frequency of the carrier wave signal Vc, and t is time.
If the modulating signal Vm and the carrier wave signal Vc are applied to the input terminals 21 and 22, a set of the transistors Q23 and Q26 and a set of the transistors Q24 and Q25 turn on and off alternately at every half cycle of the carrier wave signal Vc. On the other hand, with the modulating wave signal Vm, the transistor Q21 conducts in proportion to the amplitude Em of the modulating wave signal and the transistor Q22 conducts in inverse proportion to the amplitude Em, so that the output voltage Vo is given by ##EQU1## where K is a constant.
From the equation (2), the frequency component of the carrier wave signal does not appear at the output Vo, but only the sum and difference components of the two input signal frequencies appear, from which it is known that frequency conversion has been done.
If the voltage drop of the resistors R25 and R26 is VR, the collector-emitter voltage of the transistors Q21 and Q23 is 0.4 V and the voltage across the constant-current source CS21 is 0.4 V, the minimum operating voltage Vcc.sub.MIN of this circuit can be obtained as: EQU Vcc.sub.MIN =V(CS21)+Vce(Q21)+Vce(Q23)+VR=1.2+VR(V) (3)
Supposing VR=1 V in this equation (3), EQU Vcc.sub.MIN =2.2 V.
When the balanced modulation circuit is integrated into a battery-driven integrated circuit device, it is desirable to construct the integrated circuit device so as to operate on a supply voltage of about 1.8 V. If the arrangement mentioned above is used, since Vcc.sub.MIN is high, the dynamic range is limited accordingly.